Project Summary: B lymphocytes play a critical role in both primary and secondary immune responses to influenza and are absolutely essential for protection when animals are infected with virulent influenza viruses including H5N1 and H9N2 that are a particular concern for humans. Despite a clear requirement for B cells in immune responses to influenza, very little is known about the molecular requirements to sustain influenza-specific memory B cells and plasma cells within the respiratory tract. Likewise, almost nothing is known about the antibody-independent mechanisms by which B cells provide immune protection after infection with this respiratory virus. Therefore, the goals of this proposal are to systematically identify the signals required to induce and maintain antibody-secreting plasma cells in the lung and to determine the roles that B cells play in regulating the cellular immune response to influenza. In Aim 1, we will determine the lifespan of flu-specific plasma cells in the lung and in the bone marrow and will identify whether the lifespan of these cells is influenced by survival factors expressed in the lung, by the presence of antigen or CD4 T cells, or by the presence of other lung-tropic pathogens or pathogen-derived polyclonal B cell activators such as CpG oligonucleotides. We will also determine whether immunization in the respiratory tract promotes the formation of long-lived plasma cells within the lung. In Aim 2, we will determine whether B cells regulate the size of the CD4 T cell response to influenza by antibody independent mechanisms and will test whether cytokine production by B cells enhances CD4 T cell responses to influenza. Finally, we will determine whether immunization with protein antigens that can elicit B cell help will improve CD4 T cell responses to influenza and will improve immune protection upon secondary challenge. These experiments are important and relevant because they will increase our understanding of how to induce and maintain protective humoral and cellular immune responses to influenza. Relevance: Influenza infections cause the death of more than 35,000 people per year in the United States and during pandemic infection years, influenza has killed more than 18 million people world-wide. With the increasing likelihood that one or more of the emerging avian strains of influenza will adapt and become more infective to humans it is critical that we quickly gain a more complete understanding of how immune responses to influenza are generated and then maintained over the course of a lifetime. The experiments in this grant will identify the ways in which B cells contribute to immunity to influenza infection and will facilitate the design of more effective vaccines to this important human disease.